This project is concerned with a study of the mechanism of penicillin and cephalosporin biosynthesis with special reference to the intimate details for the construction of the thiazolidine and dihydrothiazine nuclei. We have synthesized (2S, 3S) - (4-C13) valine and have clearly demonstrated via cmr spectroscopy that it was incorporated into penicillin N by Cephalosporium acremonium with retention of configuration. In the case of Cephalosporin c, the C13- label resides exclusively in the exocyclic methylene carbon (C-17). Also, (2S, 3S) - (4-CD3) valine and (2S, 3R) - (4-CD3) valine were prepared and fed to C. acremonium. The resulting penicillin N and Cephalosporin c are currently being analyzed with respect to their deuterium content via mass spectrometry.